This year's annual flu shot will offer protection against the H1N1 flu virus, in addition to two other influenza viruses that are expected to be in circulation this flu season. A vaccine that protects against four strains of the virus will also be available, as will a high-dose flu vaccine for adults age 65 and older.

Influenza is a respiratory infection that can cause serious complications, particularly to young children, older adults and people with certain medical conditions. Flu shots are the most effective way to prevent influenza and its complications. The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months of age or older be vaccinated annually against influenza.

Here are the answers to common questions about flu shots:

Because the flu vaccine is produced by private manufacturers, its availability depends on when production is completed. For the 2017-2018 flu season, manufacturers have indicated shipments began in August and will continue throughout October and November until all vaccine is distributed. Doctors and nurses are encouraged to begin vaccinating people as soon as the flu vaccine is available in their areas.

It takes up to two weeks to build immunity after a flu shot, but you can benefit from the vaccine even if you don't get it until after the flu season starts.

New flu vaccines are released every year to keep up with rapidly adapting flu viruses. Because flu viruses evolve so quickly, last year's vaccine may not protect you from this year's viruses.

After vaccination, your immune system produces antibodies that will protect you from the vaccine viruses. In general, though, antibody levels start to decline over time — another reason to get a flu shot every year.

The CDC recommends annual influenza vaccinations for everyone age 6 months or older. Vaccination is especially important for people at high risk of influenza complications, including:

Pregnant women

Older adults

Young children

Children between 6 months and 8 years may need two doses of the flu vaccine, given at least four weeks apart, to be fully protected. Check with your child's health care provider.

You're allergic to eggs. Most types of flu vaccines contain a small amount of egg protein. If you have an egg allergy, you can receive the flu shot without any additional precautions. If you have a severe egg allergy, you should be vaccinated in a medical setting and be supervised by a doctor who is able to recognize and manage severe allergic conditions.

There are also flu vaccines that don't contain egg proteins, and are Food and Drug Administration (FDA) approved for use in people age 18 and older. Consult your doctor about your options.

You had a severe reaction to a previous flu vaccine. The flu vaccine isn't recommended for anyone who had a severe reaction to a previous flu vaccine. Check with your doctor first, though. Some reactions might not be related to the vaccine.

The flu vaccine is recommended as an injection only. While there is an FDA-approved nasal spray vaccine that consists of a low dose of live but weakened flu virus, the CDC no longer recommends nasal spray flu vaccinations because during recent flu seasons, the spray has been relatively ineffective.

The flu shot contains an inactivated vaccine made of killed virus. Because the viruses in this vaccine are killed (inactivated), the shot won't cause you to get the flu, but it will enable your body to develop the antibodies necessary to ward off influenza viruses.

The injection is usually given in a muscle in the arm. If you're between 18 and 64 years of age, you may also choose an in-the-skin (intradermal) vaccine, or you may prefer to have your vaccine delivered using a jet injector device, which uses a high-pressure, narrow stream of fluid to penetrate the skin instead of a needle.

No. The flu vaccine can't give you the flu. But you might develop flu-like symptoms — despite getting a flu shot — for a variety of reasons, including:

Reaction to the vaccine. Some people experience muscle aches and a fever for a day or two after receiving a flu shot. This may be a side effect of your body's production of protective antibodies.

The two-week window. It takes about two weeks for the flu shot to take full effect. If you're exposed to the influenza virus shortly before or during that time period, you might catch the flu.

Mismatched flu viruses. In some years, the influenza viruses used for the vaccine don't match the viruses circulating during the flu season. If this occurs, your flu shot will be less effective, but may still offer some protection.

Other illnesses. Many other diseases, such as the common cold, also produce flu-like symptoms. So you may think you have the flu when you actually don't.

How well the flu vaccine works can vary. The flu vaccine is generally more effective among healthy children age 2 and older and adults age 64 and younger. Some older people and people with certain medical conditions may develop less immunity after receiving a flu shot.

According to the CDC, in past flu seasons when the match between the flu vaccine and circulating strains of flu virus is close, a flu shot is 40 to 60 percent effective in reducing the risk of getting influenza among people of all ages.

With or without a flu shot, you can take steps to help protect yourself from the flu and other viruses. Good hygiene remains your primary defense against contagious illnesses.

Wash your hands often and thoroughly with soap and water.

Use an alcohol-based sanitizer on your hands if soap and water aren't available.

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